Transmission systems, such as those for heavy-duty trucks, which automatically shift the transmission based on throttle position, vehicle speeds, engine speeds and the like, are well known in the art. In these systems, transmission shifting can be effected in various ways. For example, the transmission system may utilize pressurized fluid to frictionally engage members to achieve a desired gear ratio. Alternatively, the system may incorporate electronic logic in conjunction with electric actuators which operate to engage and disengage mechanical clutches to achieve a desired gear ratio. These electric actuators could take the form of electric motors.
For example, United States patents relating to the automatic shifting of transmissions include U.S. Pat. No. 4,911,031, issued to Yoshimura et al., and U.S. Pat. Nos. 5,053,961 and 5,053,962, both issued to Genise and assigned to the assignee of the present invention.
In applications where a permanent magnet electric motor is utilized as a positioning device, it would be desirable to control the motor to quickly shift the transmission out of gear to the neutral position. However, the desire for this quick motion must be balanced against excessive power consumption, as well as the possibility of overheating the motor when the motor is stalled due to, for example, torque lock. Existing systems apply the same level of current to the motor during the entire preload and move to neutral motion. The speed at which the transmission can be shifted out of gear is limited by the level of current which the system can tolerate during preload.